In recent years, there has been developed an artificial photosynthesis technology which electrochemically converts sunlight into chemical substances by modeling photosynthesis of plants from viewpoints of energy problems and environmental problems. This is because even if the sunlight is converted into chemical substances at a land that is low in utility value such as a desert and is not used for production of plants and then transported to a place away therefrom, enough energy can be obtained. When the sunlight is converted into the chemical substances and stored in a cylinder or a tank, there are advantages that the energy storage cost can be reduced and the amount of storage loss is small as compared to a case where the sunlight is converted into electricity and stored in a storage battery.
As a photoelectrochemical reaction device that electrochemically converts sunlight to a chemical substance, there has been known, for example, a two-electrode type device that includes an electrode having a reduction catalyst for reducing carbon dioxide (CO2) and an electrode having an oxidation catalyst for oxidizing water (H2O), and in which these electrodes are immersed in water with carbon dioxide dissolved therein. In this case, the electrodes are electrically connected via an electric wire or the like. In the electrode having the oxidation catalyst, H2O is oxidized by light energy, whereby oxygen (½O2) is obtained and a potential is obtained. In the electrode having the reduction catalyst, by obtaining the potential from the electrode, in which the oxidation reaction is caused, carbon dioxide is reduced and formic acid (HCOOH) or the like is produced. As described above, in the two-electrode type device, the reduction potential of carbon dioxide is obtained by two-stage excitation, and therefore the conversion efficiency from the sunlight to chemical energy is low.